That’s the choice Massachusetts Democratic voters will have in three months when they head to the polls and choose a nominee for the Senate special election.

And it will be a real choice.

Rep. Stephen F. Lynch, a former ironworker elected to the House in 2001, is the most socially conservative member of the Bay State’s federal delegation. He’s set to announce his bid for Senate on Thursday, setting up a race against Rep. Edward J. Markey, a staunch progressive. Markey is the dean of the Massachusetts delegation and has held elected office since 1973.

The race will be a stark contrast in backgrounds and constituencies. Lynch, from South Boston, represents a mostly urban district that includes his home turf and has many middle- and working-class constituents who are socially conservative Democrats. Markey represents a broad swath of suburbs outside Boston that includes some urban areas, such as his hometown of Malden, but also some of the wealthiest and most liberal municipalities in the state.

Markey has the backing of the Democratic Senatorial Campaign Committee, departing Sen. John Kerry and former Rep. Barney Frank, among others. Lynch begins with some strong local organized labor support, though Markey will also have some unions on his side.

Markey supports abortion rights. Lynch is personally against abortion rights but has supported funding for organizations such as Planned Parenthood. Markey voted for the health care overhaul; Lynch voted against it.

Given their backgrounds and political positions, it’s no surprise the early narratives of Markey and Lynch bend in different directions.

“This special election should be focused on issues and ideas that matter most to the people of Massachusetts — getting dangerous guns off our street and out of our schools, building a clean energy future, and strengthening the middle class,” Markey wrote Wednesday on the influential Bay State blog Blue Mass Group.

Scott Ferson, a Lynch spokesman, said the congressman would focus on “bread-and-butter” issues in his Senate announcement: the economy, job creation and policies that benefit working families.

Ferson acknowledged that Lynch would be fighting against the Washington, D.C., establishment, but he said the congressman had never been a go-along-to-get-along member.

“He didn’t go to Washington to fit in; he went there to stand up for working people,” Ferson said.

Lynch undoubtedly begins the primary as the underdog. But Democratic insiders see a plausible path to victory for him if Markey stumbles and Lynch can unite strong union support with working-class Democrats in urban areas such as Boston, Worcester, Springfield and Lowell.

And, insiders said, progressive voters will be pleasantly surprised by how liberal Lynch is once — if — they get past his views on abortion.

Indeed, on most issues, both members are in step with their party. In votes in 2012 where a majority of Democrats voted against a majority of Republicans, Lynch voted with his party 91 percent of the time whereas Markey voted with his party 99 percent of the time.

“Calling me the least liberal member from Massachusetts is like calling me the slowest Kenyan in the Boston Marathon,’’ Lynch told the Boston Globe in 2010. “It’s all relative.’’

Each candidate will bring different strengths in the sprint to the primary on April 30.

“Stevie will be more of a street fighter, and Eddie will be more of [a TV] air warrior,” one plugged-in Boston official told CQ Roll Call.

With $3.1 million in the bank in November, Markey had a financial advantage over Lynch’s $740,000. Support from the national Democratic establishment probably means that fundraising gap will continue to grow.

While no one expects a particularly bloody primary between colleagues, their differences are expected to be accentuated.

“It will be a largely positive campaign,” said a Democratic operative who supports Markey. “But Democratic, progressive values will play a role. Lynch is out of step with those priorities, and that will be a big deal.”

“I’m hard-pressed to think of a time when a pro-life candidate has won a Democratic primary,” the operative added.

If Lynch manages to get through the primary, his anti-abortion views could add difficulty in a general election, as progressive voters might not be as inclined to schlep to the polls to support him.

On Wednesday, Massachusetts Democratic Gov. Deval Patrick set the date of the special election for June 25. In the interim, he appointed his former chief of staff, William “Mo” Cowan, to the Senate. Cowan will not run in the special election.

Former Sen. Scott P. Brown, a Republican who was defeated by now-Sen. Elizabeth Warren in November, has not said whether he will run in the special election. If he doesn’t run, the Democratic nominee is very likely to be the state’s next senator, as the bench of Republicans in the state is thin.